Maybe I was just mistaken, I haven't played in a while.  Besides the rumble pack, the only other way to find some of the stuff was to have Navi locate it for you, or blindly look for something when you don't know where it is (or use a walkthrough).  I think as long as they don't take any main component from the game by requiring an add-on, but just add to it (like completing side quests, etc), that it's perfectly fair.  One example of when they shouldn't is the much-hated Crystal Chronicles requiring portable gaming systems for multiplayer.  Bad form.

Having the rumble pack helps, to be certain.  Specifically with some of the stuff in the overworld.  There are other clues to help you if you don't have the rumble pack inserted, though.  It's a far cry from having stuff you literally can't get otherwise (i.e. that one brother of Tingle and... whatever other stuff you get in Wind Waker for having the GBA cable).  Crystal Chronicles was mostly absurd because there was really no legitimate reason to require GBAs (Zelda: Four Swords implemented their use much much better).

The funny thing is, based on sales figures, one would assume that since SO FREAKIN' MANY GBAs were sold, it would be most likely that any Gamecube owner would have one.  That turned out not to be the case fairly often, though.  Regardless, while some of the GBA->Cube connectivity was kind of fun, it was still a shot in the foot for them.